1. Field of the Invention
This invention relates to magnetic core memories and more particularly to a magnetic core memory with a crossover-free sense winding.
2. Discussion of the Prior Art
Core memories commonly in use utilize a two part sense winding which passes through a core array parallel to a given drive conductor such as the Y conductor. In order to achieve first order noise cancellation, the two parts of the winding change columns as they are crossed one or more times during each pass through the array. This affords near perfect cancellation of noises from external fields and excellent cancellation of noises generated within the core array itself.
The signal appearing across the sense winding can be represented by an equation derived in an article, "Freeman, J. R. "Pulse Responses of Ferrite Memore Cores," IRE Wescon Convention Record (1954), pp. 50-61. The equation is, ##EQU1##
Vout is the read-out voltage of the array.
VS is the switching signal output voltage of the selected core.
Vhs is the voltage output of any half select core whose output polarity on the sense winding is opposite to that of the selected core and is not cancelled by a mating core.
Vd is the difference between the average voltage output of the half selected cores whose polarities on the sense winding are the same as that of the selected core and the average voltage output of the half selected cores whose polarities on the sense winding are opposite to that of the selected core, exclusive of the two Vhs outputs (and exclusive of the selected core).
n is the number of X drive conductors or Y drive conductors in a square array.
The equation is derived for a worst case data pattern. The selected core generates a single output signal VS which appears once in the equation. In a typical bow-tie sense winding arrangement, the half selected cores appear on the sense winding in self cancelling oppositely mated pairs. However, the equation separately treats the selected core and hence its mated pair for both the X and Y partial select current must also be separately treated, we thus have the term -2 Vhs to account for these two cores. The final term accounts for the remaining half selected cores less the selected core and its unmated pair. The term is multiplied by 2 because there are both X and Y partial select currents and divided by 2 because the term represents the noise difference between a pair of cores having opposite states of magnetization.
Letting L1 represent the inductance of a core in the one state (the one will switch when read) and L0 represent the inductance of a core in the zero state, the equation can be restated as ##EQU2## where L is the average inducgance of a half selected core. The signal from the selected core may be either positive or negative (the signal is bipolar) and the worst case data pattern will be the data pattern such that ones appear on one part of the sense line and zeros appear on the other part.
While the crossovers provide excellent noise cancellation, they have inherent disadvantages. In stringing a core memory stack a large portion of the stringing time, especially in small arrays, and hence capital and labor cost is devoted to aligning the drive wire or sense wire needle with a column or row of cores. Once aligned, the needle passes rapidly through the cores. Each sense winding crossover point requires re-alignment of the needle and therefore increases stringing time and costs. Furthermore, each crossover point requires considerable space between the cores on either side of the crossover. For small arrays, this crossover space can approach or exceed the actual area occupied by the cores and thus double the required array area. The probability of damage to a core or conductor wire is also increased at each crossover.